The invention relates to an equalisation arrangement, and in particular an equalisation arrangement for use in a communications system for the correction of phase and amplitude distortion errors caused by multipath propagation effects and the like.
Communications systems employing radio, ultra-sonic, infra-red or optical transmission are well known in the art. It is a common problem in such systems, however, that under certain circumstances propagation of the transmitted signal takes place not only along one path, but along two or more paths simultaneously. When this occurs, multiple signals are received at the receiving antenna at times which depend on the length of the individual propagation paths. The result may be distortion in both the phase and amplitude of the received signal. Such a situation is illustrated in FIG. 1. Line (a) of FIG. 1 shows the situation at a time t.sub.1 as a receive antenna picks up a transmitted signal 10 of wavelength X and zero-crossing phase of 0, .pi., 2.pi., etc relative to the ordinate 11. This is a signal from a direct propagation path. Subsequently, at a time t.sub.2, the same signal but from a different propagation path arrives at the antenna and interferes with the already received signal in (a). This newly arrived signal is shown as signal 12 in line (b) of FIG. 1. The result of the superimposition of the two signals 10 and 12 is the dotted waveform 13 and this waveform can be seen both to have a larger amplitude than signal 10 and to bear a different zero-crossing phase relationship with it, namely .theta..sub.1, .pi.+.theta..sub.1, 2.pi.+.theta..sub.1, etc.
Finally, due to a further propagation path, a third signal 14 arrives at the antenna (see line (c)) and this interferes with the composite signal 13 to produce a final composite 15 having still greater amplitude, due to constructive interference, and a phase relationship of .theta..sub.2, .pi.+.theta..sub.2, 2.pi.+.theta..sub.2, etc, relative to signal 10.
Thus it can be seen that the effect of multipath propagation is to distort the amplitude and phase characteristics of the received signal with respect to the transmitted signal. Note that, for simplicity, signals 12 and 14 have been shown to have the same amplitude as signal 10; in reality, their amplitude may well be much smaller due to attenuation factors in transmission, resulting in a composite waveform 15 manifesting amplitude and phase distortions less severe than those shown in line FIG. 1.
Such distortion can be especially troublesome when digitally modulated signals are being received, as, when the distortion is severe enough, the information relating to one symbol in a data chain can interfere with the information relating to the next symbol in the chain, or even symbols further on.
It is an aim of the invention to provide an equalisation arrangement which compensates for the above-mentioned amplitude and phase distortion effects.